Steroid and Protein Regulators of Glial Cell Proliferation

Abstract

Normal development, differentiation and proliferation of tissues and cell types within multicellular organisms are stringently regulated by a complex combination of environmental factors that include systemic steroid and protein hormones, local and systemic acting growth factors, extracellular matrix components and cell-cell interactions. In most tissues, cell differentiation is accompanied by an arrest of proliferation and the maintenance of the growth arrested state is controled by specific hormonal signals which inhibit the expression or activity of growth stimulatory factors and/or induce growth suppressor gene products (Aaronson, 1991). However, for certain physiological processes, cell proliferation is neccessary to maintain populations of specific cell types. For instance, stem cells in the bone marrow provide a continuous source of hematopoietic cells in animals (Zipori, 1992). Quiescent cells will proliferate during tissue regeneration in a damaged organ, such in the liver (Fausto and Weber, 1993), whereas mammary epithelial cells are hormonally stimulated to proliferate during pregnancy and lactation (Ceriani, 1974). Finally, glial proliferation in response to brain neuronal damage has been well documented (Landis, 1994). The maintenance of these cells in the proliferative state, or the triggering of quiescent cells to proliferate, requires the selective stimulation of growth stimulatory gene products and/or suppression of growth inhibitory gene products.